My usual minimum length is 1" but I have modified this to accommodate a new installation technique/helpful hint for yourself and anyone else interested.
Whenever you install a pad eye or any other modification on the external side of the hull and use a backing nut like a nyl-lock nut or other variation, increase the length/step up to the next size of the bolt to permit the addition of another pad eye on the inside of the hull. This technique can also be used when you install rod mounts from RAM. Two of the holes on the RAM rod holder bases are the exact distance between the mounting holes on pad eyes.
Why would you do this you ask?
Simple, the answer is actually two-fold. First, the use of an additional pad eye acts as an additional waterproofing seal when a dab of LEXEL, GOOP or other sealant is used prior to installation of the nut.
Secondly, the pad eye acts as an attachment point for any gear you might stow on the inside of the hull and prevents gear from slipping into areas of the hull that are difficult to reach. Simply create a short lanyard using paracord or other lashing material and attach a carabiner or snap bolt that can hook into the pad eye.
Hope you find this useful.
Thanks for your input. I ordered the 1-1/4" #10 SS machine screws. I guess if it's too long, I'll make a cap for it. Hopefully not.
Good idea with double sided pad eyes, I'll try it out.